It looks as though Microsoft's rumoured keyboard covers with built-in batteries will arrive sometime after the company launches its new Surface line.
Citing anonymous sources, both Neowin.net and the Windows SuperSite reported recently that the "Power Cover" is definitely in the works. It will supply more battery juice to the Intel-Haswell-based Surface 2 Pro, but it'll also be compatible with the current Intel-based Surface Pro and the upcoming ARM-based Surface 2. It won't be compatible with the Surface RT.
Windows SuperSite's Paul Thurrott specifically said the Power Cover is a Type Cover-like product that's 9.75mm thick and snaps on to the Surface tablets. Microsoft's existing Type Cover is 5.33mm thick. It's unclear how much extra power the new cover will provide, but it is supposedly a "significant" battery-life boost.
The Power Covers aren't expected to ship until after the Surface 2 and Surface Pro 2. There's no word yet on pricing or availability, but stay tuned. The latest rumours have pointed to a mid-October launch for Microsoft's new line of Surfaces.